Conventionally, as a power plant of this kind, one disclosed in PTL 1 is known. This conventional power plant is applied to a four-wheel vehicle, and includes an internal combustion engine as a motive power source, a differential gear for distributing torque of the engine to left and right output shafts, a rotatable carrier member, a triple pinion gear rotatably supported by the carrier member, and hydraulic speed-increasing and speed-reducing clutches. The left and right output shafts are connected to left and right drive wheels, respectively. Further, the triple pinion gear comprises a first pinion gear, a second pinion gear, and a third pinion gear, which have pitch circles different from each other, and these first to three pinion gears are integrally formed with each other. The first pinion gear is in mesh with a first sun gear integrally formed with the right output shaft, and the second pinion gear is in mesh with a second sun gear integrally formed with the left output shaft. Further, the third pinion gear is in mesh with a rotatable third sun gear. Furthermore, the third sun gear and an immovable casing are connected to and disconnected from each other by the speed-increasing clutch, and the carrier member and the casing are connected to and disconnected from each other by the speed-reducing clutch.
In the conventional power plant constructed as above, during straight forward traveling thereof, the third sun gear and the casing are disconnected from each other by disengagement of the speed-increasing clutch, and the carrier member and the casing are disconnected from each other by disengagement of the speed-reducing clutch. Further, torque of the engine is distributed to the left and right output shafts via the differential gear. Accordingly, the carrier member, the third sun gear, and the speed-increasing and speed-reducing clutches idly rotate along with transmission of rotation from the engine. Further, during the left or right turning of the vehicle, by controlling the engagement forces of the speed-increasing and speed-reducing clutches, the distribution of torque to the left and right output shafts is controlled. Specifically, during the right turning of the vehicle, the third sun gear and the casing are disconnected from each other by disengagement of the speed-increasing clutch, and the carrier member and the casing are connected to each other by engaging the speed-reducing clutch, whereby the carrier member is decelerated. This causes part of torque of the right output shaft to be transmitted to the left output shaft via the first sun gear, the first pinion gear, the second pinion gear, and the second sun gear, so that torque distributed to the left output shaft is increased with respect to the right output shaft. In this case, by controlling the degree of the engagement of the speed-reducing clutch, the torque distributed to the left output shaft is controlled.
On the other hand, during the left turning of the vehicle, the carrier member and the casing are disconnected from each other by disengagement of the speed-reducing clutch, and the third sun gear and the casing are connected to each other by engagement of the speed-increasing clutch, whereby the carrier member is accelerated. This causes part of torque of the left output shaft to be transmitted to the right output shaft via the second sun gear, the second pinion gear, the first pinion gear, and the first sun gear, so that torque distributed to the right output shaft is increased with respect to the left output shaft. In this case, by controlling the degree of the engagement of the speed-increasing clutch, the torque distributed to the right output shaft is controlled.